Within mammals and humans there are many hemoproteins whose apparent physiological functions are quite diverse although they all contain the same prosthetic group (protoporphyrin IX) which is the active principle responsible for their unique functions. Thus, hemoglobin which is abundantly present in blood carries out the non-catalytic transport of molecular oxygen to the tissues where it is utilized for energy metabolism, and cytochrome P450 which is the major hemoprotein of liver activates molecular oxygen and catalyzes the oxidative metabolism of drugs. We have discovered that hemoglobin can fully mimic the enzymic properties of cytochrome P450 under certain conditions, and this has led us to pursue comparative studies of these two hemoproteins both in their isolated forms and in their native environments in order to learn what factors are responsible for their differential activities. The research program includes investigations of drug interactions with the two hemoproteins under various experimental conditions as monitored by physicochemical techniques including UV/visible and nuclear magnetic resonance spectroscopy.